Ok, dpuu suggested Graph, which I recommend. This does it:

#!/usr/bin/perl -w
use strict;

use Graph::Undirected;
my $net = Graph::Undirected->new;
while (<DATA>) {
    $net = $net->add_edge( split );
}

use Data::Dumper;
print Dumper($net);

__END__
0 1
1 2
1 3
1 4
2 4
4 5
[download]

Graph.pm takes care of circular references for you.

After Compline,
Zaxo